Correlation between isoform composition of the 17 kDa myosin light chain and maximal shortening velocity in smooth muscle.

The relation between the isoform distribution of the myosin 17 kDa essential light chain (LC17) and the mechanical properties of smooth muscle was investigated. The relative content of the basic (LC17b) and acidic (LC17a) isoelectric variants of the 17 kDa myosin light chain was determined in different mammalian smooth muscle tissues. The relative content of LC17b varied between muscles: rabbit rectococcygeus 0%, rabbit trachea 5%, guinea-pig taenia coli 21%, rat uterus 38%, rabbit aorta 56% and rat aorta 60%. The rate of tension development was determined following photolysis of caged-adenosine triphosphate (ATP) in skinned fibres activated with thiophosphorylation of the regulatory light chains. The half-time for force development was 0.67 s in rabbit rectococcygeus, 1.6 s in rabbit trachea, 1.13 s in guinea-pig taenia coli and 1.38 s in rabbit aorta. The maximal shortening velocity (Vmax) was determined with the isotonic quick release technique in skinned fibre preparations activated with thiophosphorylation. Vmax was 0.25 muscle lengths per second (ML/s) in rabbit rectococcygeus, 0.24 ML/s in rabbit trachea, 0.17 ML/s in guinea-pig taenia coli, 0.11 ML/s in rat uterus and 0.03 ML/s in rabbit aorta. The range of variation in Vmax between muscles was larger than in the half-time for force development. The inverse relationship between Vmax and the relative content of LC17b in the investigated muscles suggests that the type of essential myosin light chain influences the Vmax in smooth muscle.